Also adds CanvasBuffer copy constructor and refactors getUniform to return a QVariant instead of QVariantList to be more in line with the spec.

Change-Id: I626e4b41acdceae7ce8ccbcca8f9577fea09d4b7
Reviewed-by: Tomi Korpipää <tomi.korpipaa@digia.com>
Reviewed-by: Pasi Keränen <pasi.keranen@digia.com>